The present invention relates to the field of magnetic resonance imaging or "MRI".
MRI is widely used in medical and other arts to obtain images of a subject such as a medical patient. The patient's body is placed within the subject-receiving space a primary field magnet and exposed to a strong, substantially constant primary magnetic field. Powerful radio frequency ("RF") signals are broadcast into the subject receiving space to excite atomic nuclei within the patient's body into nuclear magnetic resonance, so that the nuclei spin around axes aligned with the magnetic field. The spinning nuclei generate minuscule RF signals, referred to herein as magnetic resonance signals. By applying magnetic field gradients so that the magnitude of the magnetic field varies with location inside the subject-receiving space, the magnetic resonance phenomenon can be limited to only a particular region or "slice" over the patient's body, so that all of the magnetic resonance signals come from that slice. Moreover, by applying such magnetic field gradients, characteristics of the magnetic resonance signals from different locations within the slice, such as the frequency and phase of the signals can be made to vary in a predictable manner depending upon position within the slice. Stated another way, the magnetic resonance signals are "spatially encoded" so that it is possible to distinguish between signals from different parts of a slice. After performing many excitations under different gradients, it is possible to derive a map showing the intensity or other characteristics of magnetic resonance signals versus position within the slice. Because these characteristics vary with the concentration of different chemical substances and other chemical characteristics of the tissue, different tissues provide different magnetic resonance signal characteristics. When the map of magnetic resonance signal characteristics is displayed in a visual format, such as on a computer screen or printed image the map forms a picture of the structures within the patient's body, with different tissues having different intensities or colors.
The RF excitation signals are normally applied by antennas fixed to the primary field magnet structure and arranged to provide substantially uniform excitation throughout the subject-receiving space. The RF excitation signals are provided by powerful radio transmitters. The magnetic resonance signals, which are many millions of times weaker than the RF excitation signals, can be received by antennas mounted on the primary field magnet or, more commonly, by antennas placed close to the area of the patient's body to be imaged. For example, where the patient's head is to be imaged, a loop-like coil antenna can be placed around the patient's head for reception of the magnetic resonance signals. Typically, these antennas are provided with flexible cables and connectors for attaching them to RF amplification and receiving devices incorporated in the MRI apparatus. The closely spaced RF receiving antenna provides efficient electromagnetic coupling between the antenna and the region of interest within the patient's body and thus provides efficient reception of the weak RF response signals.
By contrast, the magnet-mounted transmitting antennas normally do not provide efficient coupling to the areas of interest. That is, substantial parts of the RF power applied through such antennas are directed to regions of the patient's body other than the area of interest, and to regions of the subject-receiving space not occupied by the patient's body. To provide the desired RF excitation signal intensity within the region of interest, very strong RF signals must be applied through the transmitting antennas. This, in turn, requires expensive, powerful RF transmitters. Various proposals have been advanced to use localized antennas, similar to the antennas used for receiving the response signals, as transmitting antennas. If the RF excitation signal is applied through a localized transmitting antenna, with good coupling to the region of interest within the patient's body, the same RF field strength within the region of interest can be achieved with substantially less RF power from the transmitter. However, these proposals suffer from considerable practical difficulties. Even at the reduced power levels required, a heavy duty, well-grounded cable and connection are required to link the local antenna to the transmitter. The cables and connections are cumbersome, particularly where the antenna must be supported on the patient's body. Moreover, different transmitter coils typically are required for different regions of interest. Thus, the localized antennas must be repeatedly connected and disconnected from the transmitter during operation of the apparatus. This is burdensome to the operator because it is difficult and time consuming to connect and disconnect the heavy duty RF signal cables required for the excitation signals. Accordingly, there have been needs for MRI imaging methods and apparatus which provide better coupling of the transmitted RF signal to a region of interest, without the disadvantages associated with previous attempts to use local RF transmitting antennas.
Another difficulty encountered with conventional RF transmitting antennas mounted on the primary field magnet relates to the electrical interaction between the antenna and the remaining structure of the magnet. Typically, RF transmitting antennas have been provided as coils arranged in a plane, with the plane of the coil closely overlying a part of the magnet structure, most typically the pole piece of the magnet. To conserve room within the subject receiving space and leave a large open area for the patient, it is desirable to place the coil as close as possible to the magnet structure. However, the transmitting antenna and the magnet structure cooperatively act as a capacitor. When the transmitting antenna is arranged in close proximity to the magnet structure, a so-called "parasitic capacitance" is introduced into the electrical circuit of the transmitting antenna. This, in turn, causes problems in tuning the antenna. To provide efficient RF signal propagation, the resonant frequency of the transmitting antenna circuit must be equal to the frequency of the RF excitation signals to be sent and hence, must be equal to the resonant frequency of the atomic nuclei. The resonant frequency of the antenna circuit is inversely related to the inductance and the capacitance present in the circuit as a whole. The antenna has electromagnetic inductance. Preferably, the parasitic capacitance of the antenna together with the inductance of the antenna provide an untuned resonant frequency higher than the desired resonant frequency to match the RF excitation frequency. It is a simple matter to connect an additional capacitor into the transmitting antenna circuit so as to reduce its resonant frequency and thereby match the resonant frequency of the antenna circuit to the RF excitation frequency. However, where the parasitic capacitance and the natural inductance of the antenna, without any added capacitance, yield a resonant frequency below the RF excitation frequency, the antenna circuit cannot be tuned to the RF excitation frequency.
There has been a need for an RF transmitting antenna and transmitting antenna mounting structure which minimizes parasitic capacitance between the antenna and the magnet structure, which does not impair the other required characteristics of a transmitting antenna, and which also does not obstruct the subject receiving space to a substantial degree. This need has become progressively more acute. Higher magnetic field strengths generally yield better image qualities. Accordingly, MRI instrument designers have sought to use higher magnetic field strengths, above about 0.3 T and typically about 0.6 T. However, the resonant frequency of atomic nuclei and the required RF excitation frequency are directly related to the strength of the primary magnetic field. As the RF excitation frequency increases, the requirements for parasitic capacitance become progressively more stringent.